A liquid chromatography/mass spectrometry apparatus (LC/MS) generally comprises a liquid chromatograph which separates components in a sample in the time direction, and an atmospheric pressure ionization mass spectrometer which ionizes, under a substantially ambient pressure atmosphere, the components in the liquid sample eluted from the liquid chromatograph column, and performs mass analysis thereof. There are several techniques for atmospheric pressure ionization methods used to ionize the components in a liquid sample, with electrospray ionization (ESI), atmospheric pressure chemical ionization (APCI) and the like being commonly used.
Specifically, in the case of ESI, the tip of a nozzle connected to the end of the column of a liquid chromatograph is arranged facing inside an ionization chamber which has a substantially ambient pressure atmosphere, and a high voltage of about several kV is applied to the tip part of said nozzle. The liquid sample which has reached the tip part of the nozzle undergoes charge separation due to the action of the electric field generated by this high voltage and is atomized by being pulled apart mainly by Coulomb attraction. Liquid drops produced in this manner collide with air inside the ionization chamber and are reduced in size, and at the same time the solvent inside the liquid drops is vaporized. In this process, the component molecules in the liquid drops acquire an electric charge and fly out from the liquid drops, whereby gas ions are generated.
Furthermore, in the case of APCI, a needle electrode is arranged in front of a nozzle tip arranged facing inside an ionizing chamber. Carrier gas ions (buffer ions) generated by corona discharge from the needle electrode are chemically reacted with drops of liquid sample atomized by heating in the nozzle, thereby ionizing component molecules in the sample.
In this way, ESI and APCI differ in the principle of ionization, and also differ in the types of samples suitable for ionization. Thus, in common LC/MS, an ionization interface unit (hereinafter referred to as “ionization probe”) is provided for APCI and for ESI, and either ionization probe can be mounted in the housing in which the ionization chamber is formed. Furthermore, ionization probes have been developed which can simultaneously perform both ESI and APCI, as described in Non-Patent Literature 1.
In an ionization probe as described above, generally, nebulizer gas, i.e. nitrogen gas or the like, is used to spray the liquid sample into the ionization chamber, but there are ionization probes known in the prior art wherein high temperature drying gas is used in addition to the nebulizer gas in order to promote the gasification of solvent from the atomized sample drops (see Patent Literature 1, etc.).
FIG. 4 is a simplified cross-sectional view of this sort of conventional ESI ionization probe.
In this ESI ionization probe 100, a narrow diameter metal narrow tube 101 through which is supplied a liquid sample eluted for example from the column outlet end of a liquid chromatograph, and a nebulizer gas tube 102 through which nebulizer gas (for example, nitrogen gas) is supplied, are formed in a coaxial double circular tube shape, and a high voltage of about several kV is applied from power supply unit 105 to the tip part of metal narrow tube 101 and nebulizer gas tube 102. Moreover, an assist gas nozzle 103a is provided further to the outside of the nebulizer gas tube 102 so as to surround the nebulizer gas tube 102, and assist gas (for example, nitrogen gas) which is heated by a heater 104 provided inside assist gas tube 103 when it passes through that tube 103 is supplied to the assist gas nozzle 103a. 
Since a high voltage is applied to the tip part of the metal thin tube 101 and the nebulizer gas tube 102, a substantially annular electric insulation member 106 is arranged between the tip side (the bottom side in FIG. 4) and the base side (the top side in FIG. 4) in the nebulizer gas tube 102, and a substantially annular electric insulation member 107 is also arranged between the metal thin tube 101 and the nebulizer gas tube 102, thereby ensuring electrical insulation. The electrical insulation members 106 and 107 are members made for instance from synthetic resin or rubber.
As shown by the arrows in FIG. 4, a liquid sample is supplied to the metal narrow tube 101, and upon reaching the tip part 101a thereof, the liquid sample is charged by the action of the biased electric field formed in the vicinity of the tip end part 101a. The nebulizer gas supplied to the nebulizer gas tube 102 is discharged in the same direction as the flow of liquid sample from the nebulizer gas outlet 102a. With the assistance of nebulizer gas, the charged liquid sample turns into micro-drops and is discharged into the ionization chamber, which has a substantially ambient pressure atmosphere. The charged drops which have been thus discharged collide with the surrounding gas particles and are reduced in size, and the solvent in the drops is vaporized, in which process, ions derived from the sample components are generated.
Assist gas heated to about 400° C. to 500° C. by a heater 104 is discharged from assist gas nozzle 103a in the same direction as the flow of liquid sample and nebulizer gas. Therefore, high temperature assist gas surrounds the liquid sample spray stream, the liquid drops are efficiently heated, and gasification of solvent is promoted. As a result, ionization efficiency increases. Furthermore, the assist gas flow suppresses the spreading of the liquid sample spray stream, so the generated ions derived from the sample components can more easily reach the ion introduction unit of the mass spectrometer without scattering. As a result, more ions derived from sample components can be introduced into the mass spectrometer, for example, a quadrupole mass filter, making it possible to increase the analysis sensitivity.